The present invention relates to an automatic focusing device which automatically focuses the optical system of a still video camera, a silver halide film type of camera, binoculars and the like.
There are two kinds of systems in a conventional automatic focusing system, one is an active system in which a photographic object is irradiated with signal light such as auxiliary light, and the reflected signal is utilized, and the other is a passive system in which an image signal is utilized in some form.
In the passive system, the signal sent from a photographic object is utilized, and the obtained result is accurate, so that the passive system is widely used. In the case of a video camera, an image sensor (CCD) for the purpose of photographing is provided in the camera, so that automatic focusing can be conducted by utilizing an image signal obtained from the image sensor. A still video camera is provided with an image sensor in the same manner as the aforementioned movie video camera. Accordingly, when an image signal obtained from the image sensor is utilized, automatic focusing can be conducted accurately and advantageously without installing an optical system exclusively used for automatic focusing.
The conventional automatic focusing system used for a movie video camera in which an image signal is used for automatic focusing, is applied to a still video camera in the following manner.
The level of a high frequency component contained in an image signal becomes maximum at the best focusing position, and as a defocusing amount increases, the level of the high frequency component becomes low.
Utilizing the aforementioned phenomenon, the operation is conducted in such a manner that: at each time photographing is conducted, the scanning operation of a picture-taking lens is conducted from a very close position to an infinite position at a plurality of positions a.sub.0, a.sub.1, a.sub.2 . . . ; high frequency component f.sub.0 contained in the image signal is extracted by a band-pass filter during the scanning operation; levels of the high frequency components are successively compared so that the position where the level of the high frequency component becomes maximum can be found, for example position al is found; and the picture-taking lens is moved to position al. FIG. 8(A) shows the composition of hard ware of the aforementioned conventional example, FIG. 8(B) shows each scanning position of the picture-taking lens, and FIG. 8(C) shows the level of a high frequency component at each scanning position.
There are several problems in the aforementioned conventional automatic focusing system, and the problems will be described later.
In the case of the active system, it is necessary to provide an optical unit exclusively used for automatic focusing.
In the case of the passive system, the picture-taking lens must be moved from a very close position to an infinite position in order to find the best focus position, so that it takes a long time.
In order to improve detecting accuracy, the band width of the band-pass filter may be made narrow. When it is made too narrow, the frequency component is hardly contained in the narrow band depending on a photographic object, so that output of sufficient intensity can not be obtained.